Little Isabelle Kate Horn was born after Joanna endured an 18-hour labour, with Jeff sharing the news on his Facebook page.
'Welcome to the world my gorgeous baby girl Isabelle Kate Horn,' he wrote'
'She was the last but best thing to happen to Joanna and I in 2017. She was born at 7:40pm on Saturday the 30th of December 2017. We can’t wait to share the rest of our lives with you. Xo.'
Joanna also shared a sweet family photo from her Instagram page welcoming the newest addition to their family.
'Welcome my absolutely perfect baby daughter, Isabelle Kate Horn. We already love you unequivocally and unconditionally,' she wrote.
Horn made headlines earlier this year when he defeated ex-world welterweight title holder Manny Pacquiao - a feat which he admits pales in comparison to the birth of his daughter.
'Nothing really compares to that, you can't beat this moment,' he said to The Courier Mail.
'She's gorgeous. I had tears in my eyes and even just before talking about it makes me start tearing up.'
Congrats to Jeff and his family!